1. Field of the Invention
The present invention relates to motor controls, or more particularly to a system for controlling the operating speed and direction of a three-phase induction motor. Still more particularly, the invention relates to a system for controlling the operating speed and direction of a three-phase induction motor used in corrugated box printing equipment. The invention particularly finds use for bringing rotary printing cylinders precisely into register for printing sheets of corrugated cardboard in a machine for printing such corrugated boxes.
2. Description of the Prior Art
During the processing of corrugated container blanks, corrugated cardboard sheets are passed through successive processing stations such as printing and die-cutting. A printing operation can require one, two, three or more printing stations and may employ one or more colored inks. Each printing station is normally comprised of rotary printing machines operating to impress an image on a cardboard box blank. A rotary printing machine typically comprises two opposed cylinders turning in opposite directions at the same circumferential speed. The series of printing stations are rotationally timed relative to each other so that each sheet passes through each station at a particular instant. As the various printing stations rotate and contact successive sheets, each station is intended to print at the correct position on the sheet. In this way, colors are superimposed on top of each other on the sheet to form the final product, e.g. a multicolored printed container blank. Should any printing operation not be performed at the correct position on the sheet, then the image is said to be out of register, thus producing an inferior processed sheet. The need to maintain accurate registration of paperboard sheets in the production of color printed container blanks has become more critical with higher production speeds and the demand for higher quality printing and multi-color graphics. The present invention is concerned with improving the accuracy of position registration when a multiplicity of sheets are moved successively through a plurality of printing stations.
The printing cylinders which are attached to rotary printing machines are rotated by means of three phase AC induction motors via an arrangement of gears designed to ensure that the circumferential speed of the print cylinders corresponds to the arrival of each sequential corrugated sheet to be printed. In these arrangements, a feed device introduces one sheet after another at constant intervals theoretically equal to that of the printing cylinder, i.e., one sheet for each revolution of the cylinder. Unfortunately, the accuracy of the arrival of the sheets to the printing cylinders is generally insufficient for the accuracy required for correctly positioning the printing on the sheets. This is due to slight irregularities in the sheet, displacement of the sheets of cardboard in the feed mechanism, and random slippage of the drive rollers of the printing machines. When the sheets are not presented strictly in register with the printing cylinders with the required precision, considerable waste results.
In the usual case, the printing cylinders are connected to a sheet feed device by means of a planetary gear train in which the satellite carrier is rotationally connected to a braking motor-reduction gear unit. The action of this motor-reduction gear unit should then enable the user to offset the print cylinders so as to bring them into register with the sheets to be printed. In this regard, it would be desirable to operate the braking motors at either a high speed during the printing operation or at a very slow speed, i.e. on the order of 1 Hz to effect print cylinder position adjustments. Unfortunately, the brakes on exiting braking motors tend to engage at such slow speeds making minute cylinder adjustments difficult or impossible. The present invention provides an improved motor control circuit which better controls the speed and direction of the motor turning the printing cylinders. The motor can operate in either the forward or reverse directions and at either high operational speed, or at a very slow position adjustment speed after brake release. The apparatus according to the invention makes it possible to reduce or avoid waste by presenting the print cylinders for the printing operation in precise synchronization and with images on the sheets in register. An important feature of the invention is to provide improved three phase induction motor and circuitry system which is retrofittable into existing printing machinery. Motor powering and control is accomplished using only three conductors. This is done by powering the motor with single phase current and simultaneously controlling the speed and direction of the motor via three slip ring contacts already present on such existing printing machinery.